Because of the better environmental compatibility and better utilization of regenerative energy sources, FAME has been used as fuel in Diesel engines and FAEE has been used as fuel for spark-ignition engines to an increasing extent for a number of years. FAEs primarily are obtained from vegetable oils and animal fats, such as rape-seed oil, coconut oil, palm oil, soybean oil, fish oil, tallow or the like, by transesterifying the same in at least two interconnected reaction stages, each comprising a mixer, in which the triglycerides are transesterified in a mixing operation by adding CH3OH or C2H5OH and an alkaline catalyst, and in a downstream settler, in which the mixture produced is physically separated in a demixing operation into a light phase chiefly containing FAE and into a heavy phase chiefly containing the C3H5(OH)3 formed during transesterification. The C3H5(OH)3 formed during transesterfication is dissolved in excess CH3OH and separated from the CH3OH in a rectification column. In a counterflow washing column, CH3OH and C3H5(OH)3 are washed out from the phase containing CH3OH; if necessary, the methyl ester can additionally be subjected to a distillation. To obtain pharmaceutically acceptable C3H5(OH)3, the C3H5(OH)3 containing the impurities of the fat or fatty oil used are supplied to a downstream purification stage as described in “Fatty Acid Technology”: company brochure of Lurgi AG of 03/2005.
EP 0523767 B1 describes a process for transesterifying an oil or fat with CH3OH or C2H5OH in the presence of an alkaline catalyst in liquid phase to obtain FAME or FAEE and C3H5(OH)3 in at least two reaction stages, which each comprise a mixing reactor and a separator for separating a light phase rich in ester and a heave phase rich in glycerol. Oil or fat as well as CH3OH or C2H5OH and an alkaline catalyst are charged to the mixing reactor of the first reaction stage, and these substances are mixed with each other intensively. The mixing reactor of the first and each further reaction stage is charged with CH3OH or C2H5OH and a catalyst and with the light phase rich in ester, which is obtained in the preceding reaction stage. The total amount of CH3OH or C2H5OH supplied to the mixing reactors lies in the range of 1 to 3 times the stoichiometrically necessary amount. As alkaline catalysts, NaOH, KOH or CH3NaOH can, for example, be used in a concentration of generally 0.1 to 1 wt-%, based on the amount of oil or fat to be transesterified. The heavy phase rich in glycerol, which is separated in the separator of the second to last reaction stages, is at least partly recirculated to the mixing reactor of the first reaction stage. By washing with water containing 0.5 to 5 wt-% of acid, for instance, HCl or citric acid, the alkaline catalyst contained in the light phase rich in ester is neutralized, and the formation of soaps thereby is prevented. In a downstream water washing, the water-soluble substances contained in the light phase rich in ester, above all CH3OH or C2H5OH and C3H5(OH)3 and the neutralized catalyst are washed out. By means of a centrifuge, the suspension formed thereby is separated into FAME or FAEE with a water content of up to 2 wt-% and into a phase rich in water, which substantially contains CH3OH or C2H5OH and C3H5(OH)3. Since the FAME or FAEE usable as fuel in internal combustion engines should contain not more than 500 mg of water per 1 kg of FAME or FAEE, the FAME or FAEE finally is dried, e.g. in a vacuum plant, and then discharged from the process as marketable product. The CH3OH or C2H5OH thermally expelled from the phase rich in water is charged to the mixing reactor of the first reaction stage, and the C3H5(OH)3 left in the phase rich in water is obtained by evaporating the water.
A disadvantage of the process described above is that when washing the phase rich in ester, the soaps dissolved in non-separable C3H5(OH)3 in the light phase rich in ester, which are formed as undesired components in the alkaline ester hydrolysis of the triglycerides, are decomposed to free fatty acids (FFA), which can no longer be separated economically from the FAME or FAEE. According to the European standard EN 14214, the content of FFA in FAME is limited to a maximum acid number of 0.5 [mg KOH/g]. When using vegetable oils or animal fats with greatly varying raw material qualities, the maintenance of this acid number is not always ensured.